Airborne wind turbines are not connected to high structures fixed to the ground such as towers and poles. The flow energy of the wind is converted by the tethered aircraft into mechanical and electrical energy. The advantages of such systems are mainly in the fact that the high energy supply and the high uniformity of the wind at high altitudes, e.g. above 100 m, is possible with less material and at lower costs. Wind turbines which have a rotor on a ground-fixed structure are rarely realized higher than 200 m total height for both technical and economic reasons with the current state of technology. The masses and costs of the foundation and tower structure form a significant part of the expense which is almost entirely eliminated with airborne wind turbines. This makes it possible to lower the relative cost of airborne wind turbines compared with mast or tower-mounted wind turbines by designing the system for lower wind nominal speeds or higher nominal load availability. This leads to an equalization of the wind power supply and reduces expenses in the area of memory technology and distribution networks when used at inland locations.
There are several different concepts for such airborne wind turbines. Aircraft which are already converting wind energy into electrical energy and transfer this energy using a current-carrying tether to the ground, e.g. from US 20100295303 are well-known. Furthermore, there are concepts in which a mobile ground station is drawn by the aircraft on a trajectory or route on the ground, e.g. European patent specification EP 2075 461B1, as well as concepts in which a rotor located on the ground with a vertical axis is set into rotation by meaning of a towing aircraft with a tether of fixed length.
In rotor-driven wind turbines, the surface loads are typically 100-150 kg/m2, which must be eliminated via the bending torque in the rotor hub. The dimensioning factors here are, in addition to the average static loads, especially the changing bending torques at the wing root due to the wind gradient and the dead weight as well as load peaks from the tower dam and gusty winds. Here, wings of fiber composite design with performance-related masses of 5-15 kg/kW for small wind turbines and 10-25 kg/kW for megawatt turbines are used. This is associated with a surface weight of 20-60 kg/m2 for small and 50-150 kg/m2 for large systems, so that the size and growth potential of this design is naturally limited. Alternative designs can be with tethered wings or screen designs. Tethered wings are typically designed for surface loads between 30-60 kg/m2 and have a weight of approx. 100 kg/m2 including fuselage and control surfaces. The screen designs used especially in the sport sector are typically designed for surface loads between 3-10 kg/m2 and have a surface weight of approx. 0.1-0.2 kg/m2.
Tethered wings are roughly divided into textile designs which get their shape retention in the following ways:                (i) Differential pressurization resulting from the inflow into the blades (ram pressure):        
Ram pressure wings are used in parachutes and paragliders and sport kites as well as in wind propulsion sail systems for ships (sky sails) and in the development of airborne wind turbines. In this design the flow-induced pressure difference between the stagnation point and along the profile in the flow field is utilized. On the outer surfaces of the profile opened at the stagnation point, lower compressive forces are exerted than in the interior of the wing. The advantage of this design is possibly not having to use any rigid structural elements resulting in a minimal weight. The wings or screens unfold independently with the buildup of the inflow representing an increase in safety, in particular for paragliders, e.g. after a possible collapse. The disadvantages of this system are: (a) The easy collapse of the wing when there is no inflow also at the start because there are no rigid elements; (b) the need for a finely branched tethering for the load transfer, which leads to high air resistance and thus an aerodynamically inefficient wing and (c) reduced or missing efficient retrieval operation. A ram pressure wing with very low or negative angles of approach and accordingly low lift and drag coefficients cannot be flown due to the migratory ram pressure point, the special tethering and fluctuating inflow in turbulent air. Thus during recovery in the yoyo operation, virtually as much electrical energy is consumed as in the traction phase. For permanent applications, including those that do not involve the yoyo operation, the durability problem of seam connections and fabric take center stage.                (ii) Closed membrane parts under internal pressure (so-called tube kites): In water sports, tube kites (ii) have become popular because they can also be started easily even after a water landing. The tubes also allow a load concentration on the pressurized elements. Disadvantages of tube kites are, for example: Constant pressurization of structural elements is expensive, relatively heavy and prone to defects in the technical implementation. The design loses its rigidity in the event of possible leaks. Active pressurization to compensate for leakage increases the weight, energy consumption and costs in an undesirable manner. The famous sail-like designs also tend to flutter under certain inflow conditions which would affect the reliability and durability. The recovery operation is better realizable but is nevertheless also only possible in a limited manner in this design.        (iii) Rigid structure based on primarily fiber composites: Rigid structures ensure the best aerodynamic properties, where in aircraft construction and in the classical use of wind energy, the best lift/drag ratio, i.e. the best ratio of lift to drag is usually decisive. The drawbacks of known wing systems with rigid structure are as follows: The use of, e.g. a glider-like aircraft comes with high weights. The wings are so heavy that they cannot be started at wind speeds in the operating area of the system without additional tools. The lower switch on limit is relatively high with these wings, so less electricity is produced in the low wind range. The costs for wings of this construction are relatively high due to the materials used and manufacturing expenses. A combination of rigid and flexible design in the form of hang-gliders and delta wings is also known. Here, better lift/drag ratios can be realized with a structure that can be dismantled and is therefore transportable but only has surface loads of 7-10 kg/m2 and wings sizes below 20 m2.        